Package-on-package (PoP) configurations comprising subassemblies that are electrically bonded with one another and mounted one on top of the other are known as packages. The lower subassembly is in this case referred to as the bottom package, while the subassembly mounted on the lower subassembly and bonded on it forms the top package.
The lower subassembly comprises a packaged first semiconductor element, in particular a semiconductor chip of the flip-chip type, on a wiring carrier. The wiring carrier is often formed as a ball grid array substrate (BGA substrate), together with a redistribution layer, known as an interposer.
Soldered onto this lower subassembly is the upper subassembly. This comprises a packaged second semiconductor element, likewise generally a semiconductor chip, on a further BGA substrate encapsulated in what is known as the molding compound.
BGA substrates of this type that are used for bonding and subassembly attachment have regular arrangements of solder balls on their underside. These solder balls are fused on in a soldering process in an oven heated to the required temperature, and thereby represent the electrical and mechanical connection to the bonding area respectively located underneath. In the case of the upper subassembly of the package-on-package configuration, this is in particular the connection between its substrate and the redistribution layer of the lower subassembly. The lower subassembly is in the same way bonded with a circuit board on which the entire package-on-package configuration is finally to be anchored.
For performing soldering processes of this type, first a pasty flux is stamped onto the area onto which the BGA substrate is to be mounted, or a solder paste made up of a flux and small solder balls embedded in it is used. The solder balls in this case even out irregularities in the respective underlying surface in an electrically conductive manner. The required soldering temperature is generally about 220-240° C. for lead-containing solders or about 260° C. for lead-free solders.
In the case of package-on-package housing configurations, the flux is first stamped onto the redistribution layer and then, in a further method step, the solder balls are stamped onto the flux. The solder balls are initially held by the adhesive force of the flux, until in the subsequent fusing process they fuse with the solder areas of the interposer or with those on the surface of the leadframe as the wiring carrier of the bottom package.
In the case of customary prior-art connecting techniques of this type, however, it is problematic to separate the lower subassembly from the upper one because, if an additional adhesive bond is not provided between the surface of the semiconductor element in the lower subassembly and the underside of the redistribution layer, it is not possible to determine precisely in advance whether in this separating operation the lower subassembly will actually be detached from the upper one or the lower subassembly will be torn apart at the connecting points between the redistribution layer and the leadframe. For this reason, it is not possible to do without an adhesive bond between the semiconductor element and the redistribution layer of the lower subassembly.
On the other hand, this adhesive bond is accompanied by additional production problems. So far, standard adhesives have been used for the adhesive attachment, adhesives which thermally cure and at the same time form a rigid mechanical connection when they are introduced into the soldering oven. This often has the result that there is only inadequate contact between the solder balls mentioned and the soldering areas assigned to them. The redistribution layer is virtually no longer movable in its position and it is no longer possible for height tolerances or irregularities to be evened out. This often leads to inadequate soldered connections between the redistribution layer and the leadframe.